Soil pH has emerged as a key predictor of soil microbial community composition, and soil pH is also central to chemical assessments of soil for soil health and the modeling of soil’s biogeochemical processes. The traditional or standard method for determining soil pH prepares a one-to-one water-to-soil slurry, in which a glass pH electrode is immersed. Despite the standardization of this method, its meaning is unclear to soil ecologists interested in the pH organisms actually experience in their lives. Among bacteria, this external pH directly governs the production of ATP, rotation of flagellar motors, membrane transport systems, and energy expenditures to maintain a neutral internal pH. Water content of a soil slurry is far from a normal soil water content, and therefore the standard method introduces significant artifacts in our estimates of soil pH. In situ soil pH, alternatively, requires a suite of novel or revivified methods of measurement of proton activity of soil solution at normal water content, which can be 5 times and up to 20 times more acidic than the proton concentration of a settled soil slurry supernatant. We have measured soil pH simulating in situ conditions of unsaturated soil water content and an atmosphere of 2-3% CO2 for a range of soils of Wisconsin to compare to standard soil pH values. We hypothesize that, because external pH is necessary to the functions of bacterial membranes, that in situ soil pH values will even more strongly predict soil microbial community composition than standard soil pH values. If this is the case, and in situ soil pH cannot be easily derived from standard soil pH values, we will widely recommend that investigators of soil bacterial ecology consider adopting a measurement of in situ soil pH that is more representative of their studied system.